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Talking Point with Carlton Hall

Penetration Testing is still not being used to its full potential and this is all about trust and confidence in the data

Cone Penetration Testing (CPT) is commonly used as part of a ground investigation to reinforce borehole data and laboratory testing and, if it does not reconcile with the findings from more conventional techniques, it is often viewed with scepticism.

Unfortunately, this means that the true value CPT can bring to the development of geotechnical solutions is sometimes missed. A well-designed and controlled CPT programme, coupled with engineering experience, can deliver an accurate, high resolution primary dataset which can then be used with confidence in ground modelling and geotechnical design activities.

An obvious benefit of CPT is that it is fast and produces almost instantaneous results. The broad range of equipment and techniques available means that more than ever before, the method can be used on sites with difficult ground conditions and access constraints - not just soft soils and open fields.

To raise data confidence and reap the full benefits from CPT, it is vital to maintain a high level of quality control in the field as errors in raw data acquisition can easily affect subsequent geotechnical interpretations. By simply following a number of basic steps results can be improved and end-user uncertainty reduced.

“Gone are the days where CPT was an ‘add-on’ to conventional ground investigation”

Properly maintained equipment is crucial as most data errors are caused by poorly serviced or carelessly deployed equipment. Worn cone tips, sleeves, data cables and load cells that have been subjected to prolonged strain are all likely to generate error. Cones should be calibrated prior to commencing work. After each test, cone geometry, damage and data outputs should checked before the next test is carried out. The primary eff ort here is avoid an
increase in data uncertainty and a reduction in data confidence.

Cone wear observations, plus tip and sleeve measurements, should be made by the operator routinely, and to achieve high quality pore pressure data the piezocone should be resaturated and used filters swapped for fresh, fully saturated ones between every test. In extreme environmental conditions, such as on very cold mornings, care should be taken to allow cones to reach the ambient temperature before use and even preloaded before the first test in some cases.

Of course, once data is collected, it needs to be interpreted. There is a vast array of software available for this purpose but ultimately, nothing beats experienced human intervention, which is vital for validating characteristic data points, layer boundaries and soil behaviour type. While this is often labelled “interpretation”, much of this stage is in fact a data audit.

Gone are the days where CPT was an “add-on” to conventional ground investigation. The method should, in fact, play a key role in any integrated soil investigation programme, by delivering an accurate, high resolution dataset (1,000 data points per 10m push) for use with confidence in the development of ground models and subsequent geotechnical design solutions.

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